1. Field of the Invention
The Invention relates to gas analyzers and, more particularly, to a process gas analyzer and method for analyzing a process gas carried in a plant section.
2. Description of the Related Art
US 2012/0236323 A1 discloses a method and a process gas analyzer.
In the case of gas analyzers operating in accordance with the transmitted light method, the light from a light source is guided through a gas to be analyzed and subsequently detected. The light can be generated wavelength selectively and detected in broadband fashion (laser spectrometer, for example), or it can be generated in broadband fashion and detected wavelength selectively (nondispersive infrared (NDIR) gas analyzer, for example). In the case of in-situ process gas analyzers, such as those known from US 2012/0236323 A1, DE 10 2013 213 730 A1 or EP 1 693 665 A1, the light source and the detector are normally accommodated in different measuring heads that are mounted on process flanges on diametrically opposed sides of a plant section containing or carrying the process gas to be measured (for example, an exhaust gas line, a container, or chimney). In order for the light source and the detector to not come into contact with the often aggressive, hot and dust-laden process gas they are arranged behind windows. The window closes one end of a purging pipe that connects with its other open end into the gas-carrying plant section and is purged with a purge gas. The purge gas is chosen such that it has no cross-interference effect on each gas component to be measured, i.e., its spectral absorption lines lie outside the absorption lines of the process gas used for the measurement. The purge gas issues from the open ends of the purging pipes located opposite one another, which means that the measuring path for the absorption measurement of the process gas is determined by the gap between the open ends of the two purging pipes.
The higher the purge gas flow rate, the more effectively the windows can be kept free from contaminants from the process gas. In this situation, the purging rates can vary depending on the application in a range from a few liters per minute up to several hundred liters per minute. When cylinder gas is used, however, there are correspondingly high costs associated with a high consumption of purge gas. Thus, for example, nitrogen is often used as a purge gas for the measurement of oxygen. In cases in which ambient air is suitable as the purge gas, variable moisture content levels may result in cross-interference effects with gas components to be measured.
The gas analysis is based on the specific light absorption of the gas component to be measured and the absorption is dependent on the product of the concentration of the gas component and the absorption path or, in the case of low concentrations, is approximately proportional thereto. Consequently, the measurement is interfered with by the purge gas flowing into the measuring path between the purging pipes located opposite one another and partially displacing and mixing with the process gas at that location. In addition, the inflowing purge gas can change parameters, such as pressure, flow and temperature of the process gas, which affect the light absorption. This leads to the result that the effective absorption path (measuring path) in the process gas to be measured does not match the spacing of the open ends of the two purging pipes but may deviate and vary therefrom to an unknown degree.
The measurement error caused by the purging has hitherto been reduced in that a correction factor for the change in the effective measuring path and/or an offset for a possible effect on the concentration of the purge gas was defined for a constant process constellation. This only functions as long as the process and purging conditions (purge gas concentration, pressure, temperature, volume flow rate) are constant.
In the case of the process gas analyzer known from above-mentioned US 2012/0236323 A1, the purging pipes are separated briefly from the purge gas feed system via a switchable valve and subsequently completely filled with process gas by using a pump or a fan to introduce the process gas into the purging pipes instead of the purge gas or to extract purge gas present in the purging pipes and to replace it with the process gas flowing afterwards. The effective absorption path (measuring path) relevant to the determination of the concentration of the gas component to be measured is determined by the fact that the known distance between light source and detector is multiplied by the ratio of each absorption detected when the purging pipes are filled the one time with the purge gas and the other time with the process gas. Here, the effect of the absorption caused by the purge gas on the measurement is not taken into consideration. The process gas comes into contact with the windows protecting the light source and the detector. The operation of filling the purging pipes with the process gas and subsequently refilling with the purge gas can be performed repeatedly if required, but this does interrupt the current measurement each time.
From EP 1 693 665 A1, it is known to compensate for the effect of the absorption caused by the purge gas on the analysis of the process gas by the fact that after flowing through the purging pipes the purge gas is withdrawn from the pipes and analyzed in a separate measurement channel. The result of the purge gas analysis is subtracted from the result of the process gas analysis. For the separate measurement channel, one part of the light generated for the analysis of the process gas is branched off and, after irradiation of a measuring cuvette through which the collected purge gas is passed, is separately detected. The design effort involved is therefore correspondingly great.